This invention relates to pitching and sequencing techniques as applied to rotating apparatus having radially extending load carrying elements spaced apart from one another. The invention more particularly relates to modulating noise produced by the rotating load carrying elements, e.g. individual tire tread design elements contacting a road surface.
As a tire rolls, its individual tread design elements cause air disturbances upon impact with the road, creating a spectrum of audio frequencies broadly referred to as tire noise. Each design element causes a pulse, itself devoid of tone; however, the interaction of all of the pulses creates what is interpreted as the noise. For any given tread design there is associated a particular quantum of energy. Thus, while it is impossible to reduce the amount of energy released by a tire as it strikes a road surface, it is possible and has been the goal of many investigators to reduce the amplitude of the noise producing peaks as displayed by the frequency spectrum of the tire and to distribute this energy over a wide frequency band, i.e. to approach "white noise."
Various methods have been employed in the past to reduce noise generated by tires, particularly tires having transverse tread portions such as snow tires. A common expedient has been to use a variety of different pitch lengths for the tread design elements. It has also been common to utilize random or sinusoidal sequencing of the pitches in an attept to modulate the objectionable noise producing frequencies. While representing an improvement over unpitched (i.e. equally spaced) tread designs, certain significant drawbacks are apparent. One such drawback is that the tire molds become increasingly more expensive to manufacture as the number of different sipe elements increases, requiring a large expenditure for forming and bending dies to produce the necessary sheet metal stampings. Also, the randomness of the pitching sequence requires the use of relatively large, e.g. 180.degree. mold segments which increases the cost of the hand carved models from which the molds are made. Another drawback with these traditional pitching methods is that the tread frequency, equivalent to the product of the tread frequency harmonic and the rotation speed of the tire, is prevalent as a relatively high amplitude naked peak creating an objectionable noise.
It has been another characteristic of the prior art to employ mathematical methods for tuning out certain critical frequencies of a tire's noise spectrum by usually using an iterative process adapted to be solved with the aid of a computer program. The method customarily involves shifting particular tread elements to shift the corresponding critical peak frequencies to an adjacent "hole" frequency, thus leveling out and modulating the noise spectrum. Such a method is described in the publication. "Quieting Noise Mathematically -- It's Application to Snow Tires," S.A.E. paper No. 690520.
The state of the art is further exemplified by the tire tread construction technique shown in U.S. Pat. No. 3,023,798 to Moore et al.
It is a primary object of the invention to reduce noise generated by rotating apparatus having radially extending load carrying elements which produce audible noise frequencies, namely by utilizing a particular pitch ratio between load carrying elements.
It is another object to modulate noise produced by rotating apparatus having load carrying elements by pitching the load carrying elements and utilizing sequences of these pitches of such length and sequential positioning to modulate the load carrying element frequency harmonic produced upon rotation of the apparatus.
It is still another object to utilize a minimum number of pitch lengths, preferably two, for use in a tire tread element to reduce cost and improve the frequency spectrum produced by a tire impacting a road surface.
These and other objects are met and the disadvantages of the prior art are overcome by utilizing the apparatus and method of the present invention as more particularly described hereinafter.